Fixed #20828 -- Allowed @permission_required to take a list of permissions

Thanks Giggaflop for the suggestion.
Test that django.views.static.serve() generates 404 status codes.

Also, change tests to be based on SimpleTestCase.
Expand testing of Test LiveServerTestCase static files serving.
clarified misleading wording about squashing commits
Merge pull request #1464 from evildmp/squashing_commits

Clarified misleading wording about squashing commits
Fixed #20890 -- Added missing import in class-based view docs.

Thanks André Augusto.
Fixed #13518 -- Added FILE_UPLOAD_DIRECTORY_PERMISSIONS setting

This setting does for new directories what FILE_UPLOAD_PERMISSIONS
does for new files.

Thanks jacob@ for the suggestion.
Fixed #17778 -- Prevented class attributes on context from resolving as template variables.

Thanks KyleMac for the report, regebro for the patch, and Aymeric for the test.
Added missing release notes for older versions of Django
Unlocalize line numbers and ids in debug 500 view.

While using USE_L10N, line numbers and IDs were printed as comma (or
locale equivalent) separated values.

Thanks Kronuz for the report and intial patch.

Fixes #20861.